More transmission speeds enable the car to shift more smoothly because there is not as dramatic a change between gears. More speeds also improve fuel economy.

Other automakers also are moving toward transmissions with more speeds.

Chrysler Group plans to sell its Jeep Grand Cherokee and Dodge Dart with nine-speed transmissions this year. Jaguar Land Rover also will put one into its Range Rover Evoque SUV. Both Chrysler and Jaguar Land Rover are buying their transmissions from ZF Friedrichshafen, a German supplier, Sullivan said.

The GM-Ford program is designed to trim the expense of transmission design and reduce development time.

"Engineering teams from GM and Ford have already started initial design work on these new transmissions," said Jim Lanzon, GM vice president of global transmission engineering.

The automakers didn't say how much they are spending to design the new transmissions, which are expected to make it into vehicles in about four years.

"The goal is to keep hardware identical in the Ford and GM transmissions. This will maximize parts commonality and give both companies economy of scale," said Craig Renneker, Ford's chief transmission engineer. "However, we will each use our own control software to ensure that each transmission is carefully matched to the individual brand-specific vehicle DNA for each company."

Although the two largest U.S. carmakers battle constantly for auto sales, they do cooperate with each other and other companies on occasional projects. This new agreement is the third time in the last decade that GM and Ford have collaborated on transmissions.

Joint transmission development makes sense for the automakers because both have broad and similar product lineups that include cars, trucks and SUVs, Sullivan said. The previous ventures developed six-speed transmissions for front-wheel-drive cars running in about 8 million GM and Ford products worldwide, including the Ford Fusion, Edge, and Escape and GM's Chevrolet Malibu, Traverse and Chevrolet Cruze.

"I don't think anyone expected six speeds to become the bottom of the food chain so quickly," Sullivan said. "It just goes to show that these guys are hungry for any extra tenth of a mile improvement in fuel economy."